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Why does guanine pair with cytosine in DNA?

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Diagram of a DNA polynucleotide chain showing linked deoxyribose sugars, phosphate groups, and nitrogenous bases (adenine, guanine, cytosine, thymine), with an inset comparing ribose and uracil in RNA. — Nucleic acid A portion of the polynucleotide chain of deoxyribonucleic acid (DNA), with an inset showing the corresponding pentose sugar and pyrimidine base in ribonucleic acid (RNA).

Guanine (G) pairs with cytosine (C) in DNA because they are complementary nitrogenous bases capable of forming three hydrogen bonds with each other.12

These hydrogen bonds occur between specific atoms in the bases that have a high affinity for electrons, creating an electrostatic attraction that connects the two strands of the DNA double helix. The physical span of the G:C pair is nearly identical to that of the adenine-thymine (A:T) pair, which allows the two strands of the DNA molecule to bridge the sugar-phosphate backbones uniformly. This specific, stable pairing mechanism is essential for the accurate replication of DNA and the reliable transmission of genetic information.12